(1) Field of the Invention
The present invention relates to a heat exchanger. The invention is applicable to any type of heat exchanger where heat from a first fluid stream is exchanged with heat from a second fluid stream.
(2) Description of Related Art
The invention has particular application to a recuperator which enables the hot gases leaving a high temperature source such as a furnace or gas turbine to heat the incoming air. Such a recuperator is used in the engine disclosed in FIG. 4 of WO 94/12785.
In this engine, a countercurrent recuperator is used to preheat cold isothermally compressed air for use in a combustion chamber using expanded exhaust gas from the combustion chamber. This engine can be made to work using a conventional recuperator from gas turbine technology (such as the Solar Mercury 50). However, the pressure and temperature of the exhaust gas of the engine of WO 94/12785 can be greater than in a gas turbine. For example, the exhaust gas pressure of the engine is 5×105 Pa (5 bar) as opposed to atmospheric for a gas turbine. The air entering the recuperator will, for example, be at 2×106 Pa (20 bar) for a gas turbine and 1×107 Pa (100 bar) or higher for the engine. The “hot” end of the recuperator (i.e. the end at which the hot exhaust gas enters and the heated air leaves) may be 750-800° C. for the engine as opposed to 500-600 ° C. for the gas turbine. The temperature difference between the “hot” and “cold” ends of the recuperator will also be greater for the engine with the cooled exhaust gas leaving the “cold” end at a temperature of typically 250-300 ° C.
Therefore, although a conventional recuperator is suitable for use with the engine, it is designed to operate with optimum efficiency at very high flow rates and relatively low pressure. The present invention aims to provide a heat exchanger which operates most efficiently at higher pressures and lower flow rates.
CH 195,866 discloses a heat exchanger having a duct inside a pressure vessel and a number of pipes passing through the duct. Small holes are provided in the wall of the duct in order to equalise the pressure across the duct. While this arrangement is effective to reduce or eliminate the stresses arising from a steady state, spatially uniform difference in the pressure across the duct walls, it does not address the effects of various other stresses acting on the duct. Firstly, there is a stress on the duct walls which arises from the steady pressure drop within the tube bundle and which causes a spatially non-uniform pressure difference across the duct walls. This could be overcome by arranging the small holes along the length of the duct to equalise the pressure differences at various locations along the duct. However, this leads to a flow along the space outside of the duct which will prevent this space from operating adequately as an insulator hence reducing the efficiency of the heat exchanger. A second source of additional stress arises from pressure pulsations which may be present as a result of flow transients, which may either be part of normal operation or may be the result of fault conditions. The heat exchanger of CH 195,866 is unable to accommodate these conditions and is therefore not suitable as a modern high pressure heat exchanger.